Tachometer using an eddy current signal generator



E. WENK Jan. 11, 1966 TACHOMETER USING AN EDDY CURRENT SIGNAL GENERATOR 4 Sheets-Sheet 1 Filed May 29, 1962 Jan. 11, 1966 E. WENK 3,229,202

TACHOMETER USING AN EDDY CURRENT SIGNAL GENERATOR Filed May 29, 1962 4 Sheets-Sheet 2 32 xn L 35 311 FIG. 11

ty L1 Q2 FIG.12 I

E, WENK Jan. 11, 1966 TACHOMETER USING AN EDDY CURRENT SIGNAL GENERATOR 4 Sheets-Sheet 3 Filed May 29, 1962 FIG. 13

FIG. 15

Jan. 11, 1966 E. WENK 3,229,202

TACHOMETER USING AN EDDY CURRENT SIGNAL GENERATOR Filed May 29, 1962 4 Sheets-Sheet 4 FIG. 17

FIG. 18

assignor to siemensaschuckertwerke Aktiengesellschaft, Berlin-Siemensstadt, Germany, a wlllfl fation of Germany Filed May 19, 1962, Ser. No. mason C aim P ity, pplic G ma y, Ju 1 3 Claims. 61. 324-70) My invention relates to methods and apparatus for analog-measurement of rotary speeds.

According to known methods and apparatuses of this type an angle of rotation is translated into pulses whose number, within a given interval of time, serves as a measure for the rotating speed. Such digital methods have limited or no applicability in certain cases, particularly where a continuous measuring and recording of greatly variable speeds is required. Such recording requires socalled analog measuring methods. These should be capable of representing the rotating speed in a stepless and sufficiently inertia-free manner with a definite, preferably linear, correlation by another physical magnitude such as an electric voltage. Heretofore such analogrneasurements have been accomplished by electric generators that convert a rotating speed into a proportional direct voltage. Such tachometer dynamos in the form of :unipolar or commutator-type machines often exhibit disadvantages due to the necessity of taking the measuring voltage from live electric contacts. At high speeds these live contacts may cause arcing or sparking at the brushes, excessive heating, or excessive frictional wear. However, at low rotating speeds and correspondingly low voltages the accuracy of the measuring result is greatly impaired by contact resistances that cannot be taken into account accurately. When using tachometer dynamos, the measuring apparatus must furnish not only the electric power output required for measuring purposes but also'the power to overcome the often consider-able frictional losses. This may result in appreciable and often uncontrollable measuring errors. Similar difficulties are encountered with the conventional centrifugal force tachometers. Also, like the known eddy-current tachometers, .these do not readily permit employing the speed measuring device for remote measuring or remote control purposes.

It is an object of my invention to devise methods and means for performing analog-measurements of rotary speeds that avoid the above-mentioned deficiencies.

According to my invention, I employ a rotor structure which constitutes the object whose speed is to be measured, or which is connected with the object to rotate together therewith; and I subject the rotating rotor structure to the magnetic field of a stationary magnet in order to induce electric currents in the rotor structure in dependence upon the rotating speed; and I employ at least one secondary magnetic field strength caused by the induced currents, as an analog measuring quantity indicative of the speed to be measured.

Aside from a kinematic exchange of the inducing and induced components, this method of the invention differs fundamentally from the principle of the known eddycurrent tachometers. In such eddy-current tachometers the electrodynamic torque occurring between a magnet and a current-conducting part serves as a measuring magnitude. This torque is not utilized in the method and apparatus according to my invention. According to my invention a secondary magnetic flux as caused by induced currents is drawn upon for furnishing an output magnitude, preferably an. electric voltage, proportional to the speed being measured.

United States Patent.

3,229,202 Patented Jan. 11, 1966 According to another feature of my invention, the above-mentioned secondary magnetic field strength is employed as an input magnitude for the measuring or sensing stage of a regulating or control circuit.

According to still another feature of my invention, the secondary magnetic field strength is measured by re-v sponse to a magnetic flux caused by that field strength. This can be done for example by having the resulting magnetic flux act upon Hall generators, magnetic-field responsive ohmic resistors, saturable ferromagnetic circuits or magnetic circuits operating on the transductor, that is, saturable reactor or magnetic amplifier principle, individually or in combination. According to another, alternative feature of my invention, however, the secondary magnetic field strength is utilized by compensation With a separately excited field strength, preferably with the aid of automatic compensation. Thus the magnetic flux caused by the two field strengths will vanish. The compensating adjustment then necessary is indicative of the speed being measured.

The invention can also be applied in practice by hav ing a rotating machine part which primarily serves a different purpose or otherwise happens to be available, such as a hub for example, employed as a rotor structure which is subjected to the above-mentioned stationary magnetic field. Thereby electric currents are in: duced in the machine part during rotation thereof in order to generate a secondary magnetic field strength in a stationary magnetic system for the purpose and with the result already explained above. By thus applying the invention to existing machinery, the otherwise necessary bearing and coupling means for a separate rotor structure are avoided and the measuring of rotary speed according to the invention only requires the addition of stationary parts.

The above-mentioned and other objects, advantages and features of my invention, said features being set forth with particularity in the claims annexed hereto, will be apparent from and will be described in the following with reference to the embodiments of apparatus according to the invention illustrated by way of example on the accompanying drawings, in which:

FIGS. 1, 2 and 3 illustrate an apparatus according ,to the invention in which the speed measurement is elfected by utilizing secondary magnetic field strengths that weaken a primary magnetic flux. FIG. 1 is a section along the line II in FIG. 2 or the line I.I in FIG. 3. FIG. 2 is a lateral view; and FIG. 3 is a cross section alongthe line IIITIII in FIG. 1.

FIGS. 4, 5 and 6. show an apparatus according to the invention in which a measurement is effected by utilizing secondary magnetic field strengths outside of the magnetic circuit in which the primary magnetic flux occurs. FIG. 4 is a sectional view along the line IV-IV in FIG 5 or along the line IV?IV in FIG. 6.

FIG. 5 is a sectional lateral view, the section being taken along thev line VV in FIG. 4; and FIG. 6 is a cross section along the line VI VI, FIG. 4.

FIGS. 7', 8 and 9 show an apparatus in which iron cores for primary and secondary magnetic fluxes are combined and permit measuring the secondary fluxes for the resulting magnetic field strengths without influence of the primary flux upon the measuring operation. FIG. 7 is a front view taken in the direction of arrow VII shown in FIG. 8. FIG 8 is a section along the line VIII-.VIII in FIG. 7; and FIG. 9 is a cross section along the line IX- IX in FIG. 7.

FIGS. 10, 11 and 12 show a modification of a measuring apparatus according to the invention in form of a magnetic bridge arrangement which involves superposition of the magnetic fluxes for compensation of a secondary magnetic field, strength that in each case requires only one Hall generator or equivalent magnetic-field responsive sens-or member.

FIGS. 13, 14, 16, 18 and 19 illustrate respective electric circuit diagrams relating to the various apparatus shown in the preceding illustrations; and

FIGS. 15 and 17 are schematic and sectional views of two further embodiments respectively.

The apparatus according to FIGS. 1, 2 and 3 comprises a fixedly mounted permanent magnet I joined at its respective poles with pole shoes 2 and 3. The magnet produces a primary magnetic flux which passes through a circular metal disc 4 mounted on a shaft 5 to rotate together therewith. The magnetic flux path is closed through iron yoke parts 6 and 7. During rotation of the disc 4, the magnetic field induces therein eddy currents which are accompanied by a secondary magnetic field. The magnetic field strength of the secondary field has two components. One component acts directly in opposition to the primary flux and thereby weakens the so-called longitulinal flux e The second component produces the transverse flux b whose path in parts 2 and 6 is schematically indicated in FIG. 2 by broken-line arrows. Another transverse flux (not indicated on the drawing) passes through the parts 3 and 7 in the reverse direction. By suitably dimensioning the cross section of the iron traversed by the transverse fluxes, these fluxes can be made to magnetically saturate the iron. The saturation then further weakens the longitudinal flux (denoted by full-line arrows in FIG. 1) passing through the same cross sections.

The longitudinal flux is measured by means of a Hall generator 8 located between the yoke parts 6 and 7. The Hall generator consists in known manner of a semiconductor wafer for example of indium antimonide (InSb) or indium arsenide (InAs) and may have rectangular shape. During operation the Hall plate is traversed by constant current flowing between terminal electrodes that extend along the short edges of the rectangular wafer. When no magnetic flux passes through the wafer, two probe (Hall) electrodes located on the respective two long edges midway between the terminal electrodes have the same electric potential. However, when magnetic flux passes through the Hall plate, the two Hall electrodes show a potential dilference and hence furnish an output voltage whose magnitude (and direction) depend upon the magnitude (and direction) of the magnetic flux. A circuit of the type just described is illustrated in FIG. 14 and will again be mentioned below.

It will be understood from the foregoing that the longitudinal flux passing through the Hall plate 8, according to FIGS. l'to 3, produces a corresponding output voltage. When the longitudinal flux is increasingly weakened with anincrease in the rotating speed of the rotor disc 4, the corresponding decrease in the Hall generator output voltage is a measure of this speed.

In lieu of the Hall plate 8 which furnishes a voltage proportional to the flux qS a magnetic-field responsive resistor may also be employed. Such a resistor is shown at 8' in FIG. 13 in conjunction Withone of the'circuit connections known and available for such purposes. The illustrated resistor 8 is of the circular disc type and it has a small electrode at the center and a ring-shaped electrode around its periphery. The resistor is connected in a bridge circuit together with constant ohmic resistors R1, R2 and R3. The bridge circuit is energized from a current source S and the output voltage is amplified by an amplifier A and indicated by a voltmeter V. The resistance of the magneto-galvanic resistor 8 changes in proportion to the magnetic flux so that the voltage indi: cated by instrument V is indicative of the speed being measured.

In order to eliminate the effect of the primary magnetic field and hence to obtain only theweakening of flux 5;, relative to its standstill val-uein form of a Hall voltage,

the device according to the invention as shown in FIGS. 1 to 3 is provided with a magnetic shunt for the permanent magnet 1. The shunt comprises iron parts 9 and 10 which together form a gap in which another Hall generator or Hall plate 11 is located so that the Hall plate is traversed by the magnetic flux passing through the shunt par-ts 9, 10. This flux is virtually independent of the particular magnitude of the longitudinal flux or depends upon this flux only to a negligible (namely opposing) extent. Thus the two Hall plates 8 and 11 can be connected in voltage opposition in order to jointly furnish an output voltage that is proportional to the weakening of the longitudinal flux (1: This connection of the two Hall plates 8 and 11 is illustrated in the circuit diagram of FIG. 14. The two Hall plates are traversed by constant energizing current from a source S through an adjacent or calibrating resistor R4. The voltage indicated by the instrument V is proportional to the weakening of the longitudinal flux and consequently is a measure of the speed at which the rotor disc 4 is rotating.

The same effect can also be obtained without the provision of the magnetic shunt, by subjecting the second Hall generator to a separate permanent magnet which is not connected with the magnetic measuring circuit.

Such a device is shown in FIG. 15. Illustrated at the left of the broken line is the measuring device proper which is identical with the oneillustrated in FIGS. 1 to 3' except that it does not contain the parts 9, 10, 11. The additional Hall plate 11, electrically connected as-shown in FIG. 14, is magnetically separated from the sensing or measuring device proper and is located in a magnetic circuit that comprises a permanent magnet 40. The flux in the magnetic circuit can be controlled or regulated by means of a yoke 41 which acts as a magnetic shunt.

Compensation of that particular (constant) portion of the Hall voltage that stems from the primary flux, can also be obtained by purely electrical means, namely by producing a constant counter voltage or compensating voltage with the aid of any suitable separate source of current. Thus, for example, a device according to FIGS. 1 to 3 but without the magnetic shunt 9, 10, 11 may have the single Hall plate 8 connected as shown in FIG. 16. The Hall plate 8 is energized from a first current source S through a calibrating resistor R and furnishes an output voltage corresponding to the longitudinal flux. This output voltage is connected in opposition to the compensating voltage from a potentiometer rheostat R5 energized from a second current source S2 and so poled and adjusted that the voltage difference, indicated by the instrument V, is proportional to the weakening of the longitudinal flux and consequently to the rotary speed being measured. The compensating voltage can be adjusted by means of rheostat R5. A resistor R6 prevents the compensating voltage from being short-circuited through the low-ohmic Hall plate 8.

The undesired temperature dependence of themagnet 1, or the disc 4 with respect to electrical conductance, or of the Hall generator, these dependence factors being all active in the same sense, can be compensated by providing a magnetic shunt path whose permeability decreases with increasing temperature to a greater and properly rated extent than the total flux produced by the magnet 1. Thus the utilizable flux g increases with temperature in the required manner.

A device of this type is illustrated in FIG. 17. It coror FIG, 16 it. is only, necessary thatthe series resistor Rv have a temperature-responsive resistance characteristic whichchanges the control. currennthat passes through, the Hall plate with changesiin temperature, sothat the Hall. output voltage is changed correspondingly to obtain the desired temperature compensation- Thus, in the circuit diagram of FIG. 18, a temperature-dependent, resistor R7 is serially connected in the energizing circuit of. the Hall plate 8 which receives control current from the source S through a calibrating rheostat R.

Inv the embodiments illustrated in. the drawings the rotor structure to carry induced electric currents consists of a planar and homogeneous. disc consisting ofv electrically conducting material so that eddy currents are readily generated in the disc. Forreasons, of design and stability, however, the shape and. construction of the rotor structure in. apparatus according tothe invention may be modified in various respects. The rotor structurev for example may be given a hollow conical or hollow. cylindrical shape, or it may constitute a portion of a hollow spherical shape. The rotor structure may also. consist of ferro-magnetic material such as a. laminated sheet-metalstructure or a sintered structure, andmay be provided with particular conductors or paths for the induced secondary currents, preferably in form of a. squirrel cage.

Inthe apparatusshown in FIGS. 4, 5 and 6.a primary flux is produced by the magnet 12 and the magnetic circuit for this flux is closed through a yoke 17., passing twice through the rotor disc 15 that rotates together with the shaft 16. The disc 15 appears in FIG. 6 in. dotdash line for clarity, although the disc is located in front of the plane of illustration. The secondary magnetic field strength causes weakening of the longitudinal. flux indicated in FIG. 4 by full-line arrows. Such weakening increases with increasing rotary speed of the disc. However, in distinction from the apparatusv described above with reference to FIGS. 1 to 3, the weakening of the longitudinal flux is not utilized for measuring purposes. Measurement rather is based upon the eflect of the transverse flux (FIG. 5, broken-line arrows) which is caused only by the induced currents and passes through the iron bodies 18 and 19. These iron bodies, preferably of laminated design, are symmetrically arranged with respect to, the parts that carry the primary flux as is apparent from FIG. 6. Consequently, only the transverse flux traverses the, parts 18. and 19;. This fiux is measured as described above by the Hall plate 20 which issues. a proportional output voltage. As long as in the circuit of the secondary currents the self-induction remains negligible relative to the ohmic resistance, these currents are proportional to the rotary speed. Consequently, as long as magnetic saturation does not take place, the transverse component of the secondary magnetic flux produced by these secondary currents is also proportional to the speed. The speed limit up to which such proportionality is preserved with sufficient accuracy can be placed as high as necessary by correspondingly reducing the electrical conductivity, the magnetic conductivity, or both, or the induced circuit. The direct voltage issuing from the Hall generator then is proportional to the speed up to the particular limit of the apparatus.

The primary flux can also be excited electrically, exclusively or in addition to magnetic excitation. The windings 13 and 14 according to FIG. 4 serve this purpose. The windings 13 and 14 can be supplied with adjusted constant energization from a separate source of current. However, according to another feature of my invention, it is preferable to use the control current of the Hall generator for at least additional magnetic excitation by means of the windings 13 and 14.

In this case, too, a magnetic-field responsive ohmic resistance can be used in lieu of the Hall generator 20 in analogy to the use of the galvano-magnetic resistor 8' shown in FIG. 13.

In a preferred modification of the apparatus accordng o. e t nti n. he. r n ve se. flux tel contr ls a transductor (magnetic amplifier); i.e., the transverse flux effects premagnetization of a saturable magnet circuit. In such a modification the Hall generator 20 or the core ponding gne s eltl espon i e esis can be omitted. The premagnetization by the transverse. flux .i-s sensed by a transductor winding 21 (FIG. 5). In some cases it is preferable to provide a pair of trans. ductors. Thus, according to FIGS 5 and 6, a second transductor, carrying the transverse flux is embodie y a n part .2 wi a r di s 3;- A device similar to the one illustrated in FIG. 5 by the parts 18;, 9,. 2.0 and 21 can also. be. mploy d n der. to a netically compensate the transverse field 5 In this case, the Hall generator 241 servesv only as a null indicator for the purpose of controlling a counter excitation produced by the direct-current energized winding 21, so that flux p. vanishes (compensation with self adjust. ment). Thefexcitationcurrent for winding 21 is then a measure of the secondary magnetic field strength and. thus of the rotary speed to be measured.

FIG. 19. shows the corresponding circuit diagram. The excitation current for the winding 21 is adjusted at the ohmic control rheostat R8 to such a value that the voltage indicated by voltmeter 44 becomes zero. The current indicated by the .ammeter 43 then is an indication, of the rotary speed.

The apparatus shown in FIGS, 7, 8 and 9 embodies a combination of the iron members that carry the longitudinal and transverse fluxes. The primary flux is produced by the magnet 24 and passes twice through the rotor disc 25 fastened on the shaft 24. The flux path is closed through the iron bodies 27-, 30 and 31. In FIG. 9, the disc 25 is indicated by a dot-and-dash line for clarity, although the disc is located in front of the plane of illustration. The two. last-mentioned parts are paral-. lel paths with respect to the longitudinal flux, each of them being traversed only by partial flux 5 or (15 However, with respect to a secondarily excited transverse flux qS (show-n in FIG. 7 by broken-line arrows) these parts are connected in series. The Hall generator 2-8 is mounted between the parts 27 and 30 and is traversed by the; flux difference A second Hall generator 29 is located between the parts 27 and 31 and is traversed by the sum of the fluxes By connecting the output voltages of the two- Hall generators 28 and 29 in series opposition (in accordance with FIG. 14), a resultant difference voltage is obtained that is propon tional to the transverse flux The .parts 30 and 31 may also be provided with windings (not illustrated) that afford compensating the transverse flux. Magnetic compensation is always of advantage in cases where, for the benefit of a larger extent of the linear measuring range, a suitably small self-induction in the circuit of the secondary currents is desire-d.

The apparatus schematically illustrated in FIGS. 10, 11 and 12. constitutes a magnetic bridge connection which affords performing the speed measurement according to the invention with the aid of a single Hall generator within a wide rotary-speed range and a virtually linear correlation of the Hall voltage to the speed being measured. The primary flux produced by the magnet 32 passes through the disc 33 fastened on the rotating shaft 34 and finds a closed magnetic circuit through the two magnetically parallel iron parts 35 and 36. These .parts are magnetically connected at only one location by intermediate pieces 37 and 38 through a Hall generator 39. As apparent from the drawings, the Hall generator 39 is traversed only by the sum of the two transverse fluxes assuming that the bridge arrangement is compensated with respect to the longitudinal flux which can readily be done in various ways, for example by air-gap adjustment. In this embodiment, too, the Hall generator may be substituted by another suitable member for measuring magnetic fluxes. Furthermore, as detransverse fluxes can be compensated by compensating windings arranged on the parts 37 and 38 and can thus be indirectly measured.

The methods and devices according to the invention afford the following outstanding advantages.

The frictional moment is extremely slight or entirely absent, particularly with a flying (unilaterally journalled) arrangement of the rotor structure or when using an already present machine part for this purpose.

The measuring power is not produced by the measuring object that is only controlled thereby. This minimizes the danger of falsifying the measuring results by the measuring operation, and this danger can be reduced to any desired extent by providing for correspondingly reduced primary magnetic excitation and measuring of a secondary magnetic field strength through corresponding amplification. When the measuring of the field strength is effected on the transductor principle, such amplification is inherently afforded by virtue of the invention.

The devices operate Without movable contacts and thus without corresponding limitation in rotary speed. They have a simple and rugged design, are light in weight and small in volume, requiring no maintenance because all excitation components and voltage issuing components are stationary and fixed.

When performing the measuring operation according to the invention with Hall generators, the measuring voltage obtained can be made to constitute a pure direct voltage without superimposed alternating voltage, so that the output is an analog representation of the rotary speed with respect to magnitude as well as direction. Since a Hall generator constitutes a low-ohmic source, a lowloss transmission of the measuring value through conductors of great length is available, thus facilitating remote transmission and remote control.

To those skilled in the art it will be obvious upon a study of this disclosure that my invention is amendable to a variety of modifications and can be given embodiments other than particularly illustrated and described herein, without departing from the essential features of my invention and within the scope of the claims annexed hereto.

' Iclairn:

1. A tachometer comprising a conductive rotor adapted to be connected to the object whose rotary speed is to be measured, a stationary magnet inductively interlinked with said rotor for inducing currents in said rotor during rotation thereof, a return flux yoke inductively interlinked with said rotor, a Hall voltage generator positioned in said return flux yoke, and variable electrical means linked with said yoke for opposing the flux in said yoke, whereby the input to said electrical means constitutes a measure of the rotor speed when the output of the Hall voltage generator is null.

2. A tachometer comprising a conductive rotor adapted to be connected to the object whose rotary speed is to be measured, a stationary permanent magnet inductively interlinked with said rotor for inducing currents in said r-otor during rotation thereof, a return flux yoke inductively interlinked with said rotor for measuring rotor flux, a Hall voltage generator positioned in said return flux yoke and variable electrical means inductively coupled with said return flux yoke and thereby with said Hall voltage generator for opposing the flux in said return flux yoke, whereby the input to said electrical means constitutes a measure of the rotor speed when the output of said Hall voltage generator is null, said. variable electrical means being responsive to the output of said Hall voltage generator for producing the null output thereof.

3. A tachometer comprising a conductive rotor adapted to be connected to the object Whose rotary speed is to be measured, a stationary permanent magnet inductively interlinked with said rotor for inducing currents in said rotor during rotation thereof, a Hall voltage generator having yoke members inductively interlinked with said rotor for measuring rotor flux as a measure of its speed, a second permanent magnet magnetically separated from said stationary permanentmagnet, a sec-0nd Hall voltage generator having output electrodes connected in series opposition tothose of said first Hall voltage generator and magnetically interlinked with said second permanent magnet, and magnetic shunt means for regulating the flux in the second Hall voltage generator.

References Cited by the Examiner UNITED STATES PATENTS 1/1962 Carlstein 324- 8/1962 Hummel 32434 FREDERICK M. STRADER, CHARLES W.

HOFFMANN, MICHAEL J. LYNCH,

Assistant Examiners. 

1. A TACHOMETER COMPRISING A CONDUCTIVE ROTOR ADAPTED TO BE CONNECTED TO THE OBJECT WHOSE ROTARY SPEED IS TO BE MEASURED, A STATIONARY MAGNET INDUDCTIVELY INTERLINKED WITH SAID ROTOR FOR INDUCING CURRENTS IN SAID ROTOR DURING ROTATION THEREOF, A RETURN FLUX YOKE INDUCTIVELY INTERLINKED WITH SAID ROTOR, A HALL VOLTAGE GENERATOR POSITIONED IN SAID RETURN FLUX YOKE, AND VARIABLE ELECTRICAL MEANS LINKED WITH SAID YOKE FOR OPPOSING THE FLUX IN SAID YOKE, WHEREBY THE INPUT TO SAID ELECTRICAL MEANS CONSTITUTES A MEASURE OF THE ROTOR SPEED WHEN THE OUTPUT OF THE HALL VOLTAGE GENERATOR IS NULL. 